Does the triphenylamine-based D35 dye sensitizer form aggregates on metal-oxide surfaces?

https://doi.org/10.1016/j.jphotochem.2015.01.007Get rights and content

Highlights

  • Absorption and emission properties of D35 dye-sensitized zirconia and titania nanoparticles are examined.

  • Small population of J-aggregates identified, with evidence of dye monomer to dye aggregate energy transfer occurring.

  • D35 dye monomer's excited state lifetime on zirconia is measured to be 1.42 ns.

  • D35 dye monomer's electron injection quantum yield on titania is estimated to be 0.89.

Abstract

The absorption and emission properties of the D35 organic dye sensitizer attached to zirconia and titania nanoparticles are examined. The dye-sensitized nanoparticles are prepared in the presence and absence of the chenodeoxycholic acid (CDCA) co-adsorbent to control the amount of dye adsorbed to the metal-oxide surface. The broadening of the dye's optical absorption band on the long wavelength side for layers without CDCA co-adsorbent demonstrates that J-aggregates are formed. Fast energy transfer is found to occur from the excited dye monomers to the dye aggregates. From the time-resolved fluorescence decay curves recorded for the dye attached to zirconia, the isolated dye monomer is determined to have an excited state lifetime of 1.42 ns, whereas the largest dye aggregates have shorter lifetimes (≤1.00 ns). From the time-resolved fluorescence decay curves recorded for the dye attached to titania, the isolated dye monomer is estimated to have an electron injection rate of 6 ns−1 and electron injection quantum yield of 0.89. The results provide a clear view of the light-harvesting behaviour for the triphenylamine-based D35 dye, which is commonly employed within dye-sensitized solar cells.

Introduction

Dye-sensitized solar cells (DSSCs) are attractive devices for harvesting solar energy, especially as they can be printed into colourful, flexible, semi-transparent panels, making them ideal for building-integrated photovoltaics [1], [2], [3]. Early DSSCs employed a ruthenium-based dye as the light-absorbing sensitizer attached to the titania electrode, together with an iodide/triiodide redox couple within the electrolyte [2]. Although DSSCs based on these components have reasonably high conversion efficiencies (η 11 %) [4], ruthenium's high cost and the corrosive nature of iodide/triiodide have hampered commercialization.

Recently, research into DSSCs has been reinvigorated by the improvement of organic dye sensitizers [5], [6]. The best organic dyes are designed to have electron donor and electron acceptor motifs located at opposite ends of the molecule, connected through a π-bridge motif. These organic dyes can be synthesised from low-cost precursors and usually possess higher absorption coefficients than ruthenium-based dyes. Furthermore, through the incorporation of steric substituents, certain organic dyes have been found to prevent injected electrons within the titania conduction band being intercepted by the oxidized redox species [7], [8], [9]. This insulation of the electrode/electrolyte interface has allowed one-electron redox couples, such as cobalt polypyridines, to be deployed [8], [9]. Cobalt polypyridine redox couples enhance cell performance as their high redox potentials produce larger open-circuit voltages.

The D35 organic dye was the first sensitizer to be used with a cobalt polypyridine redox couple to achieve a high DSSC conversion efficiency (η = 6.7%) and remains a popular dye sensitizer [8]. Structurally, the D35 dye contains a triphenylamine donor motif, a thiophene π-bridge, and a cyanoacrylic acid acceptor motif (Fig. 1). The four butoxyl chains attached to the donor motif are pivotal for the dye's efficacy and have been proposed to have two functions [7], [8], [10], [11]. First, when the D35 dye is bound to the titania electrode at full coverage, the butoxyl groups create a steric barrier at the electrode/electrolyte interface, inhibiting recombination. This was confirmed by the measured lifetime of the injected electrons within the titania conduction band being longer for D35 than for essentially identical dyes that lack butoxyl chains [7], [8]. Second, the butoxyl groups prevent dye molecules from aggregating. This conclusion was supported by the fact that sensitizing the electrode with a solution containing the D35 dye and the inert co-adsorbent chenodeoxycholic acid (CDCA, Fig. 1) barely improved the cell's performance [7]. Although CDCA does not act as a sensitizer, when attached to the titania surface it separates the dye molecules, preventing aggregation. This normally improves the DSSC's performance because dye aggregates have shorter excited state lifetimes than the dye monomer, decreasing the electron injection efficiency of the former species [2], [12].

In the current study, we test the accepted view that the D35 dye sensitizer does not form aggregates on the metal-oxide electrode within a DSSC [7], [8], [10], [11], [13]. Our approach involves examining the absorption and emission properties of D35 dye-sensitized zirconia and titania nanoparticles (NPs), prepared with and without the CDCA co-adsorbent. In contrast to the prevailing view, we find evidence that the D35 dye sensitizer forms J-aggregates. We characterize the photophysical properties of the dye monomer and aggregates using time-resolved fluorescence spectroscopy.

Section snippets

Experimental approach

Sensitizing solutions were prepared using a 0.2 mM D35 dye solution and a 1 mM CDCA co-adsorbent solution (both in acetonitrile:tert-butanol, 1:1). The D35 dye and CDCA co-adsorbent were sourced from Dyenamo and Sigma–Aldrich, respectively. To prepare the dye-sensitized NPs, 5 mg of either zirconia NPs (NanoAmor, ZrO2, 3%Y2O3 stabilised, spherical, 20–30 nm) or titania NPs (Sigma–Aldrich, TiO2, anatase, spherical, <25 nm) was added to 1.4 mL of the sensitizing solution (D35+CDCA samples: 0.023 mL D35

Dye aggregation on metal-oxide surfaces

Dye aggregates are formed when two or more dye molecules bind together through non-covalent intermolecular forces. Coupling of the monomers’ transition dipole moments results in new excited state absorptions that are coherently delocalized over the aggregate [18], [19]. Dye aggregates are usually defined as being either J- or H-aggregates, depending on whether their excitation band is red- or blue-shifted, respectively, from that of the dye monomer. J-aggregates are formed when the S1←S0

Conclusion

We have examined the absorption and emission properties of D35 dye-sensitized zirconia and titania NPs, with the goal of understanding of the dye's photophysical behaviour within DSSCs. Samples were prepared in the presence and absence of the CDCA co-adsorbent to control the surface-bound dye concentration. The dye's optical absorption band is broadened on the long wavelength side when CDCA is not applied, signalling the formation of a small population of J-aggregates at full dye coverage. On

Acknowledgements

This research was supported under the Australian Research Council's Discovery Project funding scheme (Project Number DP120100100). V. Dryza acknowledges an Australian Renewable Energy Agency Postdoctoral Fellowship (6-F004) and support from the University of Melbourne's Early Career Researcher Grant Scheme. The picosecond laser was kindly provided by K.P. Ghiggino and T.A. Smith.

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